1. Field of the invention
The present invention relates to integrated circuits and in particular to reducing cross-talk in integrated circuits containing photodetector devices.
2. Related art
Integrated circuits (ICs) designed as image sensors containing a photodetector device array typically suffer from cross-talk. In a typical image sensor IC, electromagnetic radiation in the visible and/or non-visible spectra enters the IC's top surface above a specific underlying photodetector device used to control a specific pixel in a pixel array used to represent a picture. Cross-talk occurs when radiation entering above one particular photodetector device is reflected or refracted within the IC structure. The reflected or refracted radiation is detected by an another photodetector device, thus causing picture distortion.
IC image sensors typically contain high quality charge coupled devices (CCDs). But it is known that even "studio quality" CCDs are susceptible to cross-talk. CMOS optical sensors currently under development may provide an enhanced "studio quality" image over images produced by present CCDs, in which case greater reduction in cross-talk will be necessary.
Cross-talk may be measured by providing a mask over a photodetector device array that allows radiation (e.g., light) to enter the IC over only one underlying device. The nearby device response is then measured and a ratio of intended versus non-intended detection is calculated. Informal industry comments have reported as high as twenty-five percent non-intended response.
FIG. 1 is a cross-sectional view of a portion of a conventional integrated circuit (IC) including a photodetector device. Cross sections of two conventional conductive interconnects 2A and 2B are depicted, along with a cross section of photodetector device 4. Radiation 6 is incident on device 4 from above.
As shown, device 4 is a conventional buried-channel charge coupled image sensor formed in a region of substrate 8 bounded by dashed lines 9 and including transparent conductive gate electrode 10 overlying doped channel layer 12. A conventional transparent insulating layer 14 overlies substrate 8 and device 4 formed in region 9.
Also shown are conventional patterned conductive interconnects 2A and 2B, each having the same cross-sectional structure. For interconnect 2A, a conventional patterned barrier metal layer 16A overlies substrate 8. Barrier layer 16A, as is known in the art, is typically an 800 angstrom thick titanium nitride (TiN) layer and prevents spiking and electromigration.
A conventional patterned conductive layer 18A, such as aluminum or aluminum alloy, is shown overlying barrier layer 16A. Layer 18A interconnects circuit devices in the IC, such as device 4. As depicted, sidewall 20A of interconnect layer 18A reflects radiation in the visible spectrum and also in wavelengths above and below the visible spectrum.
If interconnect layer 18A is formed of aluminum, for example, aluminum's current-carrying capability dictates that the thickness of layer 18A be at least 0.4 to 1.0 micrometers. Thus the reflective area of sidewall 20A cannot be reduced by using a layer 18A thickness less than approximately 0.4 micrometers.
FIG. 1 also shows a conventional patterned anti-reflective layer 22A on interconnect layer 18A. Layer 22A is typically TiN, and is formed during conventional photolithography processes. The reflective top surface of, for example, an aluminum layer may interfere with masking and exposing a photoresist layer (not shown) used during photolithography to pattern an interconnect. Therefore conventional photolithographic processes apply an anti-reflective layer on such a reflective surface before applying, masking, and exposing a photoresist layer. As shown, anti-reflective layer 22A remains on layer 18A after a conventional etch forms interconnect 2A.
As FIG. 1 shows, interconnect 2B has the same cross-sectional structure as interconnect 2A. Thus layers 16B, 18B, and 22B in interconnect 2B are the same as layers 16A, 18A, and 22A, respectively, in interconnect 2A. Similarly, layer 18B's sidewall 20B is analogous to layer 18A's sidewall 20A.
A known major optical cross-talk source is light or other radiation reflected from metal interconnect sidewalls within the IC, such as sidewalls 20A and 20B shown in FIG. 1. One present method used to reduce sensor cross-talk is to form a lens matrix overlying photodetector devices such as device 4. Each unique lens in the matrix focuses incident light onto a corresponding unique device in an underlying detection device matrix. However, such a lens matrix does not address the effects of light reflected from metal interconnects or other reflective surfaces in an IC. Furthermore, dark current--current flowing in a photodetector in the absence of irradiation--may provide a photon source as well. Some dark current-emitted photons may be reflected from the metal interconnect sidewalls to be sensed by photodetectors in the IC.
What is required is a way to further reduce cross-talk.